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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

I know, I know.We
have all been instructed by Arthur C. Clarke to attempt no landings on Europa.
But if you did land on Europa,
wouldn’t you like to know where to go? If you do, my graduate student, Patrick
Fischer, has a paper coming out that you probably want to read.

First, perhaps, it might be best to understand why anyone
would want to land on Europa at all. Europa – the second of Jupiter’s four
large satellites – is clearly a special place. Ever since the time of the
Galileo spacecraft nearly 2 decades ago, we have recognized that Europa’s fresh
icy surface, covered with cracks and ridges and transform faults, is the
external signature of a vast internal salty ocean. If, on a whim, you climbed
down a crack on the surface of Europa and made your way down into the ocean
(which, interestingly, might be something you actually could do; though it is
more likely you would get stuck and squeezed to death; hard to tell) and then
you figured out how to swim down to the rocky bottom something like 100 km
below the base of the ice (a depth 10 times greater than the Marianas Trench,
by the way) you would instantly be able to answer what to me is one of the most
interesting mysteries about Europa. What is happening at the boundary of the
rocky core and the ocean? The answer has profound effects on the type of world
that Europa ultimately is.

What might be happening down there? The least interesting
possibility is that the bottom of the ocean is a stagnant, inactive place:
water on top; rock on bottom; a little dissolution of the rock into the water
in between, but, otherwise, with not much going on. A world like this wouldn’t have much of a
source of chemical energy in the ocean, and it’s hard to imagine it could
support even the most elementary types of life. If you had taken all of that
effort to swim all the way to this cold dark dead ocean bottom, you might start
to ask yourself whether or not it was even worth it. The most interesting
possibility – at least the most interesting possibility that I can think of –
is that the rocky bottom of the ocean is almost like a miniature Earth, with
plate tectonics, continents, deep trenches, and active spreading centers. Think
about mid-ocean ridges on Earth, with their black smokers belching scalding
nutrient-rich waters into a sea floor teaming with life that is surviving on
these chemicals.It doesn’t take much of
an imagination to picture the same sort of rich chemical soup in Europa’s ocean
leading to the evolution of some sort of life, living off of the internal
energy generated inside of Europa’s core. If you’re looking for Europa’s whales
– which many of my friends and I often joke that we are – this is the world you
want to look for them on.

Sadly, this is not Europa

Sadly, no one is going to climb down through a crack and
then swim to the bottom of Europa’s ocean for a long long time, so this is
where landing on the surface comes in. If the chemicals that are dissolved
inside of the ocean could somehow make it to the surface, we could learn a lot
about what is going on deep inside of Europa just by analyzing a little a
sample of the surface.

OK,then, let’s go land! But where? You probably only get one
shot at a lander, and you probably don’t get to move once you land, so you had
better pick the right spot. The announcement a couple of years ago, that plumes
of water jetting from Europa’s south pole had been discovered by the Hubble
Space Telescope, seemed to have answered the question: land at the pole, and
wait for plumes to rain down upon you (or, perhaps even more easily, fly
through the plumes and collect samples without even landing!). The bad news,
however, is that the plumes now appear to be elusive at best and non-existent
at worst. Since their initial detection no one has been able to see them again.
Are they (very) sporadic? Was the initial detection an unfortunate spurious
signal that was misinterpreted? No one yet knows, but no one today is going to
count on plumes for measuring the chemical composition of the ocean.

Luckily, our new paper shows that we don’t need plumes to
sample the interior, and we even conveniently point out a potential landing
area that is large enough to easily target with your favorite lander.

First, how do you find a landing site? What we are actually
doing is simply mapping the composition of the ices across the surface of
Europa. Such mapping has been going since the time of the Galileo mission, but
with modern telescopic instruments and high spatial resolution adaptive optics
systems on large telescope on Earth, we can do a better job of making global
scale maps than the Galileo spacecraft ever could. In the earlier Galileo
mapping efforts and in our own early analyses of our own data, we concentrated
mainly on dividing the surface of Europa into an ice component and a non-ice
component and then trying to figure out what the non-ice component was. Like
the earlier Galileo analyses, we found that the dominant non-ice component is
sulfuric acid that is created when sulfur (ultimately derived from volcanoes on
Io!) bombards the water ice on the surface of Europa. We also found, though,
that some of the non-ice material was magnesium sulfate – Epsom salts, in fact
– which we suggested indicated a magnesium source coming from inside of
Europa’s ocean that then mixes with the incoming sulfur.

Patrick Fischer, in his new analysis, decided to take these
ideas one step further. He wanted to know if there is anything else on the surface
of Europa besides just the water ice and the sulfur products. To do so, he took
the spectra of nearly 1600 separate spots on the surface of Europa and started
looking for anything unusual that stood out. The answer was…… maybe. Staring at
that many spectra you’re bound to find something to catch your eye. He needed a
more rigorous method to group the spectra together, and eventually he developed
a very clever new mathematical tool which allows you to take an arbitrary
collection of spectra and automatically, with no preconceived human biases,
classify them into an arbitrary number of distinct spectra, and present maps of
where those materials are present on the surface. When he asked the tool to
give him to find the two most distinct spectra on the surface of Europa, he
reproduced the ice plus sulfur products distributions that had been known for
decades. When he asked for a third distinct spectrum, though, a large region on
the surface of Europa suddenly popped out as being composed of material unlike
the ice or sulfur products of the previous map.Staring back and forth between the composition map he had just made and
a geological map of the surface of Europa, he was startled to realize that he
had nearly precisely mapped out one of the largest regions of what is called
“chaos terrain” on Europa.

Mapping the composition of the surface of Europa has shown that a few
large areas have large concentrations of what are thought to be salts.
These salts are systematically located in the recently resurfaced "chaos
regions," which are outlined in black. One such region, named Western
Powys Regio, has the highest concentration of these materials presumably
derived from the internal ocean, and would make an ideal landing
location for a Europa surface probe

On Europa, "chaos terrains" are regions where the icy surface appears to
have been broken apart , moved around, and frozen back together.
Observations by Caltech graduate student Patrick Fischer and colleagues
show that these regions have a composition distinct from the rest of the
surface which seems to reflect the composition of the vast ocean under
the crust of Europa.

Chaos terrain was noticed early on in the Galileo mission as
regions which look like the surface of Europa has become cracked and jumbled
and – intriguingly – perhaps even melted in recent times. If you had to vote
for a location on Europa where ocean water had recently melted through and
dumped its chemicals on the surface, you would vote for chaos terrain. And now
Patrick had found that on large regional scales chaos terrain has a different
composition than the rest of the surface of Europa!

And what do the spectra tell us that the unique composition
of this chaos terrain is? Sadly, we can’t yet tell. To date, we have not found
unique compositional indicators in the spectra of this region, though our
search is ongoing. Our best bet, though is that we are looking at salts left
over after a large amount of ocean water flowed out on to the surface and then
evaporated away. The best analogy would be to large salt flats in desert
regions of the world. Just like these salt flats, the chemical composition of
the salt reflects whatever materials were dissolved in the water before it
evaporated. On the Earth, salt flats can contain any number of exotic salts,
depending on the surrounding rock chemistry. On Europa, the salts will tell
about the rock chemistry, too, though the rock is the material far below at the
base of the ocean.

We think, then, that we have found a giant salty
patch on the surface of Europa, and very likely the region of most recent
resurfacing and undisturbed chemistry. I have tried very hard to get Patrick to
call this salty patch Margaritaville, but he does not think that graduate
students are quite established enough to make jokes like that. I’ll make it for
him, though. And I will tell you: attempt a landing there! Margaritaville will
not only have salts that tell you about the rock-ocean interaction, but it will
also have samples of everything else that the ocean has to offer. Is there
organic chemistry taking place in the oceans? Look in Margaritaville. Carbonates?
Margaritaville. Microbes? Definitely Margaritaville. All of these are best searched
for with the types of instruments currently roving around on Mars, where you
grab a sample, put it into a machine, and read back out the chemical composition.
But don’t forget to bring the cameras along, too, just to see what else is
lying around.The jumbled and exotic icy
terrain is bound to be a spectacular site up close. You might get lucky and see
a plume shooting off into the sky in the distance. And maybe, just maybe you’ll
even find a few whale bones lying around.

Good thing about whale bones and a wider range of biominerals is that these would recognizably survive far longer from intense radiation on the surface than would organic biosignatures. Impact gardening would mix these minerals meters deep into the ice over tens of millions of years after surface emergence from the ocean. Yep, you better have a high-resolution multi-spectral imager on the lander. In case you do land on a pile of fish or whale bones that have no residual organics after long irradiation. Welcome to the field of paleoastrobiology.

As it appears that the astronomers and especially exobiologists are throwing the "l" word around again. It is suspected that Mars had an ocean(s) for 150 million years, but no evidence of life has been detected; no fossils, bones, shells or freeze-dried whales.

Just because there may be liquid water, is little fodder for optimism. The Dead Sea is mostly devoid of life, if salt is what keeps the ocean liquid beneath the estimated 1-10 mile thick ice shell, then only microbes if that can survive such hyper salinity among the other extreme conditions.

The Rosetta mission earlier this year demonstrated that a probe could fly through comet ejecta and analyze its composition, so we have the technology to fly through cryo-guysers without disturbing and possibly contaminating any world.

Unfortunately, scientists will stop at nothing if the possibility of life is present. Joking aside, what if whales were discovered; would we launch a probe to penetrate the protective ice and hunt down this creature as in Herman Melville's novel Moby-Dick, place it in a box and ship it back to Earth where it would be dissected for billions of live TV/Internet viewers? If life exists elsewhere we should study it, but not disturb it, let alone kill it for our selfish desire to do whatever it takes to achieve our goals, no matter how immoral they may be.

Mankind has such a poor record regarding the preservation of life on Earth and to trust people to do the right thing is like trusting a fox to guard a hen house.

Remember the Huygens mission to Titan about 11 years ago and the thought that life may exist there? Did you think for once that by sending a probe to another world which may harbor life, that we could inadvertently contaminate it with Earth life such as microbes or viruses? If we contaminated another world, there would be noting to stop such an outbreak.

After Titan revealed nothing, why such interest in Europa, after all there are several suspected worlds with cryo-vulcanism including Titan, Triton, Enceladus, Ganymede and Miranda? Maybe Ceres has an icy shell and a liquid ocean...so what?

Until scientists understand how life forms, we seem to be grasping as straws, just hoping to find life elsewhere, oh how exciting that would be...or not.

I believe that it's been Standard Operating Procedure to make sure that all outbound probes are sterile, precisely to prevent contaminating any planet (or moon) with Earth's biota. So I would expect that the Titan probe did not contaminate Titan's biosphere, if any.

But in a roundabout way, this brings up an interesting question I've wondered about. The Europa Rover will need some human piloting, just as the Mars Rovers do. Semi-autonomous rovers can only do so much; and if Europa's surface is as rough as is suggested some human input may be necessary. But from where?

I've been wondering if Jupiter might be explored from Mars. It's closer, so there's less time-lag in communications and the radio signals would be stronger. It's also far enough, that there'd be no radiation problems; Jupiter's magnetic field and the particles it contains would kill any humans who got close enough.

Which brings me to my question: How could I get a picture of Jupiter, as seen from one of the Mars Rovers? If I read my digital orrery (The Sky, v. 5) correctly, Jupiter should be close to Martian opposition; rising at sunset, and setting at sunrise. A photo of Jupiter, low on the horizon at Martian twilight, should be terrific.

"I believe that it's been Standard Operating Procedure to make sure that all outbound probes are sterile, precisely to prevent contaminating any planet (or moon) with Earth's biota. So I would expect that the Titan probe did not contaminate Titan's biosphere, if any."

Although I understand that certain decontamination protocols are taken, but there will never be a 100% guarantee that any object, including people from Earth will be free of infectious disease pathogens or other forms of contamination such as chemicals.

Mankind can not decontaminate hospitals with 100% effectiveness, let alone with 100% effectiveness every time; it only takes one "germ" or other contaminant to damage or destroy an ecosystem that is ill-prepared for such foreign "assault." When Europeans came to the new world, the Indians were mostly wiped-out by germs brought over to the new world, not by war or starvation when the buffalo herds were culled.

Another issue that could arise is a condition found on Earth where invasive species dominate and/or take-over an ecosystem, thereby unleashing an environmental catastrophe.

It seems that preservation of exo-ecosystems is a low priority for the astronomy community, so maybe the time is right to form exo-world environmental lobbies and other exo-world environmental activist groups so exo-biologists, astronomers, (mining) companies and other groups will meet with resistance.

G Smith - just to say that the Huyguens probe didn't need special cleaning from Earth life because Titan is so very cold. -179 degrees C. Usually accepted that the lowest temperature Earth life can reproduce at is above -20 degrees C. There are some who think it may be possible below that but nobody would say it is possible at -179 degrees C. In addition - water there is in the form of ice and the liquid is ethane / methane.

Now there is an idea that Titan has a subsurface ocean, and - just possibly might involve cryovolcanism. If that was possible, then that could change things. I think it is still a matter for debate

http://onlinelibrary.wiley.com/doi/10.1002/jgre.20062/abstract

If you had cryovolcanism, you could potentially have habitats for Earth life also.

Anyway whatever the situation for Titan - for Europa then there is a major risk of contaminating it with Earth life. Any lander would have to be thoroughly sterilized. The radiation from Jupiter may not be enough - if there was liquid water below the surface with contact with the surface, and the contamination from the surface gets to it quickly enough - there are many microbes that can withstand very high levels of radiation. Radiodurans can withstand 5,000 grays for instance. https://en.wikipedia.org/wiki/Deinococcus_radiodurans

And just a few meters below the surface of Europa then most of the radiation from Jupiter would be shielded already.

This would have to be considered if anyone does a lander on Europa. And they've already decided that the Sagan probability approach originally used for Mars is not appropriate for Europa. Because it is so hard to figure out the probabilities.

I think myself that we simply shouldn't attempt a landing on Europa until we have examined it close up with orbiters and understand its surface properties very well. I.e. not send a lander on the next Europa orbiter mission. If we ever do it though, this seems useful and interesting research.

Your comments are quite well thought-out, however it would seem that either tidal forces from a primary world and/or radioactive element decay like that which is found on Earth would be the most likely scenario that leads to life. Most life-forms as Earthling scientists are aware of, survive in warm to hot temperatures including those found around ocean bed smokestacks or geothermal hot springs such as those in Yellowstone National Park. Living cells may freeze through or burst from extreme cold, but some simple, primitive or even complex living creatures (that have evolved to live in such harsh conditions) may tolerate or even thrive under extremely cold environments. Perhaps exo-creatures have evolved with an anti-freeze that prevents cell damage; nobody knows, hence the need for caution.

Just because a world is far from the Sun or other obvious heat source, does not mean a world is frigid through and through. Io for instance may have active lava and/or magma (flows) from aforementioned forces.

It seems more likely that life would form and thrive in warm or hot environments such as in hot springs or thermo-volcanoes, not ones involving the extreme opposite. But in either case, the moral issue is what do we do if exo-life is discovered?

To change the subject for a moment professor. I am an 81 year old retired gentleman who has just finished your book about the down-grading of Pluto. In it you talked about how moons are named and I believe you said all known moons were named. But one isn't and I'd like to know why. That moon is the one that circles the planet Earth. The Earth has a name as do the other 7 planets. But our poor Moon does not and it is one of the biggest moons in the solar system . . perhaps the biggest. but we just call it Moon. That would be like naming your dog "dog" or your aardvark "aardvark." How would you like to just be called "MAN?" How about coming up with an acceptable name for moon. If for no other reason than to make an old man happy before he dies. P. S. I really liked your book. bigbillbissonnette@gmail.com